1. Field of the Invention
This invention relates to an apparatus and method for measuring the intraocular pressure of an eye, to an applanation device using therefor, and to an auxiliary device using therewith.
2. Description of the Prior Art
Measurement of the intraocular pressure of an eye is effective in the prevention against glaucoma, for the early discovery thereof, and for the diagnosis thereof. Furthermore, such measurement is utilized for monitoring the progress of the eye which has been treated or operated for glaucoma. The possibility that glaucoma causes a loss of sight is as realistic as the possibility that diabetic retinopathy (retinopathia diabetica) causes it.
Therefore, in a plurality of ophalmological examinations, the measurement of the intraocular pressure is most routinely performed much the same as the photographing of the fundus of an eye is performed.
The history of intraocular pressure determination had its beginnings in palpation but the Goldmann tonometer as taught by U.S. Pat. No. 3,070,997 solved the problem of ocular rigidity and has since been used by ophthalmologists as the most reliable device for measuring the intraocular or tonometric pressure.
As illustrated in FIG. 1, the Goldmann tonometer has a lighting system (L) consisting of a tungsten light source (2), a consenser lens (4), and infrared-near infrared cut filter (6), an aperture (8) a blue filter (9), a projection lens (10) and a mirror (12) for projecting a blue light on a flattened plane of the eye. An applanation pickup (PU) for flattening the frontal surface of the eyeball (E) to be examined, an applanation drum (PD) equipped with a manual dial (14) for adjusting the pressure with which said applanation pickup (PU) pushes the eyeball (E), and a finder (F) through which the examiner observes the flattened surface of the eye are provided. The front surface (which is brought into contact with the eyeball) of the pickup (PU) is a transparent flat surface which is provided with two micro-prisms (16a), (16b) in close contact therewith. In measuring the intraocular pressure, fluorescein (a fluorescent substance) is instilled into the examined eye and, then, the manual dial (14) of the applanation drum (PD) is manipulated so that the eyeball (E) will be gradually pressed by the pickup (PU). The lighting system (L) illuminates the front surface of the pickup (PU) with blue light alone so that the image of the aperture 8 is formed on this surface. As the eyeball (E) is gradually pressed by the applanation pickup (PU) with increasing the pressure, the front surface of the eyeball is progressively flattened and consequently the fluorescein is pooled around the flattened area. The dispersion spectrum peak of fluorescein (FL) and the excited spectrum peak lie in the neighborhood of 490 mm and 520 mm, respectively, and as the blue light excites the fluorescein (FL), a green fluorescent ring is formed around the flattened area. This area enclosed by the fluorescent ring shows the size of the flattened surface.
In the Goldmann tonometer, when this fluorescein ring is observed through finder (F), it appears like FIGS. 2(a), (b) (c) due to the effect of prisms (16a), (16b). It is so arranged that when the diameter of this fluorescein ring is just 3.06 mm, the two semicircles (18a), (18b) are brought into contact with each other as illustrated in FIGS. 2(a) (b) (c). When the diameter is more than 3.06 mm and less than 3.06 mm, the semicircular images are as illustrated in FIGS. 2(b) and (c), respectively. This diameter of 3.06 mm was set because the ocular rigidity of the eyeball can be disregarded. Therefore, as the examiner manipulates the manual dial (14) of the applanation drum (PD) while observing the fluorescein ring image (18a), (18b) and stops increasing of the pressure at the moment when said two semicircles are just brought into contact with each other as shown in FIG. 2(a), the rotational position of the manual dial (14) can be used to read the applanation pressure value W (g). Then, the intraocular pressure P (mmHg) can be determined by multiplying this W (g) value by 10.
This principle is based on the following rationale. Assuming that the shape of the flattened area is a circle with a diameter of l (mm) and an area of S (cm.sup.2) the intraocular pressure P can be expressed by the following equation. ##EQU1##
The applanation pickup (PU) is supported by a support bar which is supported at a fulcrum in a predetermined position, and a weight is mounted movably along the support bar on the opposite side of the fulcrum of the bar with respect to the applanation pickup. By manipulating the dial by hand, the weight is moved along the support bar to vary the pressure of the applanation of eyeball by the applanation pickup at the other end of the support bar.
The fundamental principle of this applanation device is now explained with reference to FIGS. 3(a), (b).
Let it be assumed as shown in the side-elevation view of FIG. 3 (a), the weight G.sub.1 weighing W.sub.1 at distance l.sub.1 from the fulcrum is in balance with the weight G.sub.2 weighing W.sub.2 at distance l.sub.2 from the fulcrum. In this condition, the relation holds. EQU W.sub.1 .multidot.l.sub.1 =W.sub.2 .multidot.l.sub.2 ( 4)
This balance is upset when, as in FIG. 3 (b), the weight G.sub.2 is displaced by distance .DELTA.x along direction x toward the fulcrum, with the result that a counter-clockwise rotation moment M is generated. This rotation moment is expressed as follows. EQU M=W.sub.1 .multidot.l.sub.1 -W.sub.2 .multidot.(l.sub.2 -.DELTA.x) (5)
In the Goldmann tonometer, the rotation moment is varied by shifting the weight linearly so as to vary the applanation pressure to the eyeball.
However, this and analogous tonometers have the various disadvantages mentioned below. First, as will be apparent from the above explanation of the basic principle, the shape of the flattened plane is assumed to be circular. However, the precision of measurement is low when the shape is not circular as a measured eye has with astigmatism. As a procedure for correcting for this disadvantage, the applanation pickup (PU) is rotated by a predetermined angle for measuring an eye which has severe astigmatism. However, since the directionality of astigmatism differs from one subject to another, the method does not ensure an accurate measurement of intraocular pressure in all cases and complicates the measuring procedure. Moreover, unless the position of the pickup (PU) relative to the eyeball (E) are precisely controlled so that the boundary between the two prisms (16a) (16b) will pass through the center of the applanation area, the true diameter of the flattened plane cannot be measured. It causes the deterioration of the accuracy of measurement. Therefore, the position of the pickup (PU) relative to the eyeball (E) must be controlled with accuracy and this adds to complexity of the measuring procedure. In addition, owing to the bleeding of instilled fluorescein into the flattened plane, the two semicircular images (18a) (18b) observed are fairly blurred. Moreover, the diameter of the flattened plane fluctuates constantly owing to the beating of the heart. Therefore, not only a high degree of skill is required for detection of the moment at which the two semicircular images come into contact as shown in FIG. 2 (a) but also individual differences are inevitable so that the precision of measurement is adversely affected. Moreover, in the Goldmann tonometer, the intraocular pressure is measured only when the diameter of the fluorescein ring is 3.06 mm. Therefore, when the two semicircular images are shifted from the condition of contact to the condition depicted in FIG. 3 (b) or (c), the diameter of the flattened plane cannot be ascertained, with the result that the fluctuations of intraocular pressure due to the beating of the subject's heart, that is the maximum, minimum, and mean intraocular pressure, for instance, cannot be determined.
Furthermore, if it is attempted to achieve an automatic applanation with the applanation mechanism of Goldmann, the following problems are encountered. In order to vary the applanation pressure with high accuracy and control, a stepping motor may be employed. Actually, however, since the stopping motor is a device adapted to produce a rotational motion, there must be provided a mechanism for converting the rotational motion of the stepping motor to a linear motion of the weight but such a mechanism requires a complicated construction. Thus, the applanation mechanism of Goldmann tonometer is not suited to an automatic operation using a stepping motor.
Furthermore, it should be necessary to instill the fluorescein liquid into the examined eye, prior to starting the measuring operation. The following methods are applied therefor.
(1) Preserve a portion of a glass rod in the fluorescein liquid, and bring it into contact with the examined eye.
(2) Absorb the fluorescein liquid in a syringe, and instill it into the examined eye.
(3) Bring the fluorescein paper into contact with the examined eye to dissolve the fluorescein liquid, contained in the paper, by the lacrimal fluid.
However, such methods cause complexity of the preparing procedure prior to the measuring procedure. And, the delicate control of the amount of the fluorescein liquid to be instilled is rendered difficult by using such methods. Therefore, the fluorescein liquid would be instilled over an indispensable amount for measuring, and it causes dicomfort to the examined person.